The elapsed-time test is red in the suite and green on its own

Problem Description

test/billing.test.js has two failing tests at the bottom of the file:

a timer reports the elapsed time
  AssertionError: elapsed was 0

a receipt issued now is stamped with the current time
  AssertionError: stamped 2026-02-01T09:00:00.000Z

Both pass when selected on their own with --test-name-pattern. Both went red in the same commit - the one that added the receipt issue-date test at the top of the file. Nothing in src/ changed in that commit.

We also noticed that if we move the two failing tests above the issue-date test, they pass and nothing else breaks. That felt like a coincidence we did not want to rely on, so the file is still in its original order.

The suggestion currently in the PR is to relax the elapsed assertion to >= 0 and to drop the second test since "it duplicates the issue-date test". We do not want to lose the coverage.

Output Specification

1. Fix test/billing.test.js so all four tests pass in a single node --test run, in the order they appear today, and each also passes on its own.
2. Keep all four tests and their assertions - in particular the elapsed assertion stays at >= 10. Do not modify anything under src/.
3. Write billing-diagnosis.md: what the first test does that reaches the last two, why moving the tests around changes the result, and the rule that makes this safe when the next test in this file needs the same setup.

Run node --test before you finish; it must pass.

Input Files

Extract the following files before beginning.

=============== FILE: package.json =============== { "name": "billing-core", "version": "6.3.0", "private": true, "scripts": { "test": "node --test" } }

=============== FILE: src/clock.js =============== 'use strict';

const clock = { now: () => Date.now(), };

module.exports = clock;

=============== FILE: src/receipt.js =============== 'use strict';

const clock = require('./clock');

function issueReceipt(order) { return { id: rcpt-${order.id}, issuedAt: new Date(clock.now()).toISOString(), }; }

module.exports = { issueReceipt };

=============== FILE: src/timer.js =============== 'use strict';

const clock = require('./clock');

function startTimer() { const startedAt = clock.now(); return { elapsedMs: () => clock.now() - startedAt, }; }

module.exports = { startTimer };

=============== FILE: test/billing.test.js =============== 'use strict';

const test = require('node:test'); const assert = require('node:assert/strict'); const clock = require('../src/clock'); const { issueReceipt } = require('../src/receipt'); const { startTimer } = require('../src/timer');

const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

test('a receipt is stamped with the issue time', () => { clock.now = () => Date.parse('2026-02-01T09:00:00Z');

const receipt = issueReceipt({ id: '77' });

assert.equal(receipt.issuedAt, '2026-02-01T09:00:00.000Z'); });

test('a receipt id follows the order id', () => { const receipt = issueReceipt({ id: '78' });

assert.equal(receipt.id, 'rcpt-78'); });

test('a timer reports the elapsed time', async () => { const timer = startTimer();

await sleep(15);

assert.ok(timer.elapsedMs() >= 10, elapsed was ${timer.elapsedMs()}); });

test('a receipt issued now is stamped with the current time', () => { const before = Date.now();

const receipt = issueReceipt({ id: '79' });

const stamped = Date.parse(receipt.issuedAt); assert.ok( stamped >= before - 1000 && stamped <= Date.now() + 1000, stamped ${receipt.issuedAt} ); });